One type of a conventional mobile station for an automobile telephone system comprises a main unit including a radio communication circuit for receiving aural signals, various data, control signals etc. from a base station and transmitting aural signals etc. to the base station, a control circuit for controlling the radio communication circuit etc. to operate in accordance with the control signals, and a power supply for supplying electric power to the circuits; and a telephone set including an operation board with a power-on switch, a key board etc. for turning the mobile station on and off, dialling a telephone number and so on, a display for displaying information supplied from the main unit etc., a receiver and a microphone for a telephone communication, a speaker for producing aural output in accordance with audio frequency signals, and a control circuit for controlling the operation board, the display etc., to operate in accordance with the control signals, wherein the main unit and the telephone set are connected to each other by a cable such as a curl cord.
In operation, the mobile station is turned on by use of the power-on switch in the operation board, so that electric power is supplied from the power supply in the main unit to the radio communication circuit, the control circuit etc. in the main circuit, and through the curl cord to the control circuit, the operation board, the display etc. in the telephone set. Then, a caller (an operator) in a vehicle can dial a telephone number of a callee which is transmitted from the control circuit of the telephone set through the curl cord to the control circuit of the main unit, so that the telephone number is transmitted from the radio communication circuit of the main unit through antennas to the base station. Consequently, the telephone set of the caller is connected through an exchange to a telephone set of the callee. In the same manner, information stored in the base station can be available to the operator.
When operation of the mobile station is finished, a power-off signal of serial data is produced by use of the power-on switch in the operation board of the telephone set, and then transmitted from the control circuit of the telephone set through the curl cord to the control circuit of the main unit, so that the mobile station is turned off in accordance with the shutting-off of the power supply.
In the conventional mobile station for an automobile telephone system, however, there is a disadvantage that the power supply remains turned on in the main unit in a case where the curl cord becomes disconnected to separate the telephone set from the main unit under a state that the power-on switch is turned on in the operation board of the telephone set, in other words, without transmitting the power-off signal from the telephone set to the main unit.